1. Field of the Invention
This invention relates generally to ball valves and is concerned in particular with an improved top entry ball valve having an unyielding seat.
2. Description of the Prior Art
There exists a long felt and heretofore unsatisfied need for a quick-acting low-cost on-off valve having the capability of achieving ANSI (American National Standard Institute) pressure-temperture ratings. The conventional gate valve, with its metal-to-metal seating and wedging action, satisfies many of these requirements. However, because screw threads are used to force the wedging action, many turns of the valve operating handle are required to move the wedge between its fully open and fully closed positions. Thus, quick action becomes an impossibility with this type of valve.
Moderately priced 1/4 turn ball valves also achieve many of the above-stated requirements. However, such valves lack the ability to achieve high pressure-temperature ratings because they employ yieldable elastomeric seats, whose pressure retaining capabilities drop rapidly as temperatures increase.
Metal-seated ball valves also are available which again achieve many but not all of the above-stated requirements. Because such valves require extremely precise machining and assembly, their costs are extremely high and this in turn severely limits where they can be used.